1.Technical Field
This disclosure relates to an imaging method for enabling high resolution image formation at high speed, an image forming apparatus that performs such an imaging method, and a computer-readable program that enables a computer to perform such imaging method.
2.Description of the Related Art
An inkjet recording apparatus is an image forming apparatus that uses one or more liquid jetting heads and may be used as a printer, a facsimile, a copier, or a multifunction copier having functions of a printer, facsimile, and copier, for example.
The inkjet recording apparatus forms (i.e. records, prints) images by discharging ink (i.e. recording liquid) from its recording head and onto the surface of a recording medium such as a sheet of paper some other medium on which recording liquid may be applied.
An image forming apparatus may be configured to form four different types (four tones) of dots, namely, “no dot”, “small dot”, “medium dot”, and “large dot”. However, such an image forming apparatus has limited capacity to form multiple tones with recording liquid droplets in different dot sizes.
Accordingly, techniques such as the dither method and the error diffusion method have been developed for reproducing halftones by combining density gradation (intensity modulation) of a lower level than that of the original image and area gradation (area modulation).
The dither method (binary dither method) uses the value of each matrix in a dither matrix as a threshold value, compares the value of the dither matrix with the density of a pixel of a corresponding coordinate, and determines whether to output 1 (print/illuminate at a target pixel) or 0 (no printing/illuminating at a target pixel), to thereby obtain a binarized image. This method can obtain binarized data for area gradation by simply comparing the input image data and the threshold values and can perform calculations at high speed.
One example of a halftone pattern used in a halftoning process of the dither method is an orderly linear base tone (e.g. diagonal line base tone).
On the other hand, a serial type (also referred to as a shuttle type or a serial scan type) inkjet recording apparatus forms images by moving a recording head mounted on a carriage in a main scanning direction (also referred to as “main scanning”) and intermittently conveying a recording medium in a sub-scanning direction. More specifically, the serial type inkjet recording apparatus forms images by using a multi-pass method and an interlace method. In conducting the multi-pass method, a group of nozzles or different groups of nozzles scan the same area of the recording medium in the main scanning direction plural times, so that a high quality image can be formed. In conducting the interlace method, an image is formed by interlacing the same area by adjusting the amount of conveying the recording medium in the sub-scanning direction and moving the recording head in the main scanning direction plural times.
In forming an image by combining the multi-pass method and the interlace method, the arrangement order for recording dots (e.g. order of applying ink droplets, order of aligning ink droplets) can form a matrix. This arrangement of dots (matrix) is referred to as a mask pattern (also referred to as recording sequence matrix).
High quality images can be formed by utilizing the mask pattern. For example, in the inkjet recording apparatus disclosed in Japanese Laid-Open Patent Application No. 2002-96455, different groups of nozzles scan the same main scan recording area of the recording medium in the main scanning direction plural times. Moreover, the inkjet recording apparatus includes a part for forming a thinned out (pixel skipped) image in accordance with a thin-out mask pattern by scanning a recording area in the main scanning direction plural times and a recording duty setting part for dividing the same recording area in a sub-scanning direction at a predetermined pitch and setting recording duties with different values in accordance with the thin-out mask pattern with respect to each divided area.
In another example, Japanese Registered Patent No. 3507415 discloses a recording apparatus having a control part for using dot arrangement patterns corresponding to a level of quantized image data to form dots corresponding to the level of the image data on a printed medium. The control part is capable of periodically changing the plural dot arrangement patterns used for the same level of the image data, wherein the plural dot arrangement patterns used for the same level of the image data are such that within each period when the patterns are periodically used, the number of dots formed in each of the N rasters are equalized, whereas the number of dots formed in the M columns are equalized, and P, N, and M are each an integral equal to or larger than 2. The plural dot arrangement patterns periodically used for the same level of the image data are such that within each period when the patterns are repeatedly used, when the dots formed using at least one of the plural dot arrangement patterns are shifted at least two pixels in the main-scanning direction, a variation in the ratio of a printing surface of the printing medium which corresponds to a printing range for the dot arrangement pattern occupied by a surface on which dots are formed using the plural dot arrangement patterns is limited to 10% or less.
In yet another example, Japanese Laid-Open Patent Application No. 2005-001221 discloses an inkjet recording apparatus using a halftone process mask in which a linear base tone of a halftone pattern forms dots that always synchronize with the dots formed by performing a combination of multi-passing and interlacing with a serial head.
Conventionally, in a case of forming halftones with a linear base tone, the impact points where the droplets contact the recording medium tend to vary for each tone. This leads to reduction of image quality due to problems such as uneven printing results and banding.
Even with the above-described apparatuses disclosed in Japanese Laid-Open Patent Application No. 2002-96455 and Japanese Registered Patent No. 3507415, the impact points tend to vary as the mask pattern becomes larger, and uneven printing results and banding may not be adequately prevented. Thus, the problems related to use of a linear base tone in a halftone process are not adequately solved by the above disclosed techniques.
In view of such problems, various image processing methods have been contemplated for preventing image degradation even when halftone processing using a linear base tone and multi-pass printing are combined.
For example, a technique has been proposed that involves forming dots aligned in a base tone direction with non-consecutive passes to reduce image degradation caused by uneven printing results and banding.
However, in the above technique, only the dispersity of dots in the base tone direction is taken into consideration and the dispersity of dots in the sub scanning direction is not taken into consideration.
Therefore, according to the above technique, although dot dispersity in the base tone direction may be decreased, problems related to dot dispersity in the sub scanning direction are not addressed. Thus, dot dispersity in the vertical direction may be increased, and lines and unevenness may be created in the vertical direction, for example.